Electrochromism and reversible changes in the position of fundamental absorption edge in cathodically deposited amorphous WO3

• Published in: Electrochimica acta 2004-06, Vol.49 (15), p.2425-2433
• Main Authors: Krasnov, Yu.S., Kolbasov, G.Ya
• Format: Article
• Language: English
• Subjects: Amorphous WO3; Chemistry; Electrochemistry; Electrochromic efficiency; Electrodeposited films; Electrodeposition; Exact sciences and technology; General and physical chemistry; Optical band gap; Proton insertion; Study of interfaces
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2004.01.020

Amorphous WO3 (a-WO3) films have been produced by electrodeposition from a sodium tungstate-based aqueous electrolyte. Their coloration under the action of cathode current in 1N H2SO4 is accompanied by a reversible shift of ∼0.42eV in the fundamental absorption edge of the oxide towards higher quantum energies. The shift of the edge is proportional to the change in the potential of the WO3 electrode being colored and is caused by the sequential filling, by injected electrons, of levels in the energy spectrum of electronic states formed by the unoccupied d-orbitals of W6+ centers. The optical characteristics of the W5+ centers which are formed in this case (color centers of electrochromic material) depend on whether a particular tungsten atom has a double bond to the oxygen atom (WO type bond). At the initial stage of coloration, injected electrons are captured mainly by the W6+ centers that have no such bonds. Then, W6+ centers with WO bonds, which have a higher position of the unoccupied d-orbitals on the energy scale, are also filled; this is accompanied by the appearance of an extra absorption band with maximum at ∼2eV in the optical spectrum of films.